The present invention relates to helically corrugated tubing having a graduated pitch and the methods for manufacturing such tubing.
The manufacture of helically corrugated tubing is old in the art as indicated by U.S. Pat. Nos. 3,015,355 and 3,533,267. In general, a plain-walled tube having a cylindrical or other cross section is locally stressed at a plurality of points to weaken the tube and start the formation of the corrugations. The tube is installed in a twisting machine such as a lathe with one end of the tube engaged by a rotatable chuck in the headstock and the opposite end restrained against rotation by the tailstock. The chuck is then rotated to twist one end of the tube relative to the other, and at the same time axially directed forces are applied to the tube by pushing the tailstock toward the headstock. As indicated in U.S. Pat. No. 3,533,267, the rate of rotation of the tube relative to the rate of axial movement of the tailstock toward the headstock controls the shape and pitch of the corrugations.
The corrugations in tubes facilitate their use in many different areas particularly in the heat exchange field where one fluid passes within the tube in heat exchange relationship with another fluid on the outside of the tube. The corrugations in the tube wall increase the surface area of the tube per unit of tube length and also create turbulent flow inside and outside of the tube to improve heat transfer coefficients at the inner and outer tube surfaces.
Tubing units incorporating helically corrugated tubing can be formed by composites of both plain-walled and corrugated tubing as indicated in U.S. Pat. No. 3,730,229. In addition, shell-and-tube heat exchangers incorporate spiral tubing in tube bundles in order to gain the benefit of improved heat transfer coefficients in the bundle design.
In the prior art heat exchangers, the corrugations in the tubing are generally uniform from one end of the tube to the other and, correspondingly, the pitch and shape of the corrugations remain substantially the same from one end of the tube to the other. While such corrugations improve the heat transfer coefficients by virtue of the larger surface areas and induced turbulence, the cross sectional area of the tube remains unchanged as in a conventional tube and any changes in state or density of the fluid mediums are not accommodated. Increased pressure levels or velocities and backpressure are experienced.
It is, accordingly, a general object of the present invention to provide a new and novel helically or spirally corrugated tubing in which the corrugations are formed in a manner which accommodates the change in state or the natural contraction or expansion of fluids that receive or give up heat in an exchanger formed from the tubing. It is a further object of the invention to disclose a heat exchanger utilizing the new helically corrugated tubing.